Nucleophilic Substitution Reactions of Thiophenyl 4-Nitrobenzoates with Pyridines in Acetonitrile

The kinetics of reactions between Z-thiophenyl 4-nitrobenzoates and X-pyridines in acetonitrile at 55.0 degrees C are investigated. The Brönsted plots obtained for the pyridinolysis of thiophenyl benzoates are curved, with the center of curvature at pK(a) approximately 4.2 (pK(a)(0)). The Brönsted plots for these nucleophilic reactions show a change in slope from a large (beta(X) congruent with 0.64-0.72) to a small (beta(X) congruent with 0.19-0.23) value, which can be attributed to a change in the rate-determining step from breakdown to formation of a tetrahedral intermediate in the reaction path as the basicity of the pyridine nucleophile increases. This mechanism is supported by the change of the cross-interaction constant rho(XZ) from a large positive (rho(XZ) = 1.41) for the weakly basic pyridines to a small negative (rho(XZ) = -0.32) value for the strongly basic pyridines. The reactivity-selectivity principle (RSP) holds for the rate-limiting breakdown but fails for the formation of the intermediate. The aminolysis of thiophenyl benzoate with deuterated 4-chlorobenzylamine catalyzed by glymes has primary deuterium kinetic isotope effects (PKIEs), k(H(cat))/k(D(cat)) congruent with 1.28-1.62.